As an example of a gas combustion type striking tool, there is known a combustion-gas driven striking machine which is operable to: inject combustible gas into a combustion chamber sealed inside a body; stir mixed gas of the combustible gas and air inside the combustion chamber; burn the mixed gas thus stirred inside the combustion chamber to generate high-pressure combustion gas inside the combustion chamber, and act the high-pressure combustion gas on a striking piston accommodated inside a striking cylinder to strikably drive the striking piston inside the striking cylinder, thereby striking a nail supplied to a nose portion beneath the body into a steel plate or a concrete with a driver coupled to the lower surface side of the striking piston. Such combustion-gas driven striking machines is configured as a portable tool equipped with a vessel, such as a gas container, which is filled with combustible gas and is arranged inside the tool, and a battery which serves a power source for igniting the combustible gas and is attached to the tool. Thus, a work for striking a nail or a pin can be carried out without being restricted by power supply sources such as electric power or compressed air.
The gas combustion type striking tool as described above is provided with a feeding mechanism which sequentially feeds coupled fasteners housed inside a magazine toward the nose portion. In some feeding mechanisms, linearly coupled fasteners are housed within a sheath-shaped magazine, and the coupled fasteners are constantly pressed toward a side of the nose portion by a spring having a constant output, whereby, immediately after the first fastener supplied at a shooting port of the nose portion is driven, the subsequent fastener is supplied inside the nose portion.
Because the number of fasters that can be housed inside such straight-type magazines is small, some gas combustion type striking tools are equipped with a cylindrical magazine inside which fasteners coupled in a coiled manner are housed.
A feed piston-cylinder mechanism is generally used as a fastener feeding mechanism for cylindrical magazines. The feed piston-cylinder mechanism is configured such that a feed piston is slidably accommodated inside a feed cylinder and is provided with a feed pawl engagable with the coupled fasteners housed inside the magazine, whereby the feed pawl reciprocates in a nail feed direction in which the feed pawl is fed toward the side of the nose portion and in a retracting direction opposite thereto.
When the gas combustion type striking tool employs the cylindrical magazine together with the feed piston-cylinder mechanism, the feed piston of the feed piston-cylinder mechanism may be reciprocated by utilizing a spring and a pressure of the combustion gas inside the combustion chamber. More specifically, the combustion chamber may be coupled to a front portion of the feed cylinder through a gas tube so that the feed piston is reciprocated such that it is fed forwardly by the spring and is retraced by the gas pressure form the gas tube.
However, according to the aforesaid configuration, when the fastener is struck by driving the driver together with the striking piston, the combustion gas inside the combustion chamber is simultaneously fed to the feed cylinder to retract the feed piston. Thereafter, when the combustion gas is cooled so that the pressure inside the combustion chamber becomes negative pressure, the striking piston returns due to the pressure difference. Simultaneously, the pressure inside the gas feeding portion of the feed cylinder also becomes negative pressure, whereby the feed piston moves in the nail feed direction due to the spring force. At this time, since the movement of the feed piston due to the spring force is faster than the returning movement of the striking piston, there sometimes arises a phenomenon that the front fastener being fed toward the nose portion by the feed pawl of the feed piston hits the driver that is still returning. This is because, while the fastener is energized toward the nail feed direction by the spring force, the retuning force of the striking piston caused by the negative pressure of the combustion chamber is not so strong. Thus, the front fastener scraped against the driver. As a result, the driver sometimes fails to return due to the frictional resistance caused by the slidable contact.
Therefore, JP 5-72380 U discloses a technique in which a check valve is provided in the gas tube while an discharge valve is provided in the front portion of the feed cylinder. In this configuration, the feeding operation of the feed pawl can be controlled by sending the combustion gas into the front portion of the feed cylinder to retract the feed piston, holding the retracted state, and then opening the discharge valve after the completion of the striking operation to discharge the gas inside the front portion of the feed cylinder.
However, the check valve and the discharge valve are provided near the tip end of the nose portion where it is likely to be exposed to an environment in which dust such as wood chips or fine particles of concrete is attached. Thus, it is difficult to ensure the sealing property of the valves which opens and closes in such an environment. When the valve is not surely sealed, the pressure of the feed piston can not be held. In such a case, a delaying operation of the feed piston becomes uncertain, so that the problem of scraping may arise.